Recently, we discovered a protein which specifically inhibited the action of FSH in the granulosa cells (GCs). Upon characterization by molecular cloning, this protein was shown to be an insulin-like growth factor binding protein, 1GFBP-3. In the course of this study, we also isolated three novel IGFBPs and cloned their cDNAs in the rat and human, and named them IGFBP-4, -5 and -6. Our studies further yielded the findings that: (i) rat GCs of the follicles which progress to ovulation do not express any IGFBPs, whereas those which are destined to die by atresia selectively express IGFBP-4 and -5; (ii) exogenous administration of IGFBP-4 and -5 to the GC culture inhibit FSH-induced steroid synthesis; thus, these IGFBPs are potent FSH antagonist; (iii) FSH inhibits the expression of 1GFBP-4 and -5 in the cultured GCs, and degrades these IGFBPs by FSH-induced protease(s); (iv) GnRH-agonist dose-dependently stimulates the expression of IGFBP-4 and -5 in the cultured GCs, and abolishes the ability of FSH to inhibit IGFBP-4 and -5 expression and to induce their protease activity. These findings led us to propose the intriguing and exciting hypothesis that the expression of these IGFBPs may lead to atresia by virtue of their ability to antagonize FSH action. The challenge is to prove this hypothesis. The ultimate goal of this proposal is to explore what controls the timing and level of IGFBP-4 and -5 expression during folliculogenesis and how they regulate the expression of these IGFBPs at the transcriptional and post-transcriptional levels in the rat GCs. Three specific aims are proposed. SPECIFIC AIM #1: To characterize the temporal pattern of expression of IGFBP-4 and -5 mRNAs and proteins in non-dominant cohort follicles during the process of atresia as it occurs physiologically over the estrous cycle, and to investigate how selectogenic and atretogenic ligands modulate IGFBP-4 and -5 mRNA and protein expression in vivo. SPECIFIC #2: To explore the control of the IGFBP-4 and -5 gene expression in the GCs at transcriptional and post-transcriptional levels in vitro. SPECIFIC AIM #3: To characterize the gene promoters responsible for controlling basal and regulated levels of IGFBP-4 and -5 transcription. The findings from the proposed studies may shed new light in understanding the mechanisms by which follicle selection and atresia are regulated; and thus, they could have new and important implications for fertility and infertility in women.